Tuesday, June 15, 2021

in-focus

Blockbuster snowstorm to strike New York, Philadelphia


A high-impact winter storm is set to take aim at the Northeast beginning late Sunday, with the heaviest snow likely from Philadelphia to New York northward to the Boston metro area.

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Widespread snow totals nearing or exceeding a foot are possible from Philadelphia to Boston, with 18 inches or more not out of the question in New York, depending on any influx of relatively mild air from the Atlantic Ocean.

The same storm system has already brought eight inches of snow in Chicago and will drop a healthy dose of snow all the way to the Canadian border on Sunday.

The low-pressure area that will bring the heavy snow, strong winds and potential for coastal flooding to the Northeast will spend Sunday getting organized in the Mid-Atlantic region, where snow was falling in the nation's capital. Washington was poised to receive more snow in one day than the city has seen in two winters.

The winter storm has a history of spawning wild weather across the Lower 48, first driving an atmospheric river ashore in California last week with extreme snow totals topping 100 inches in the Sierra Nevada. From there, it brought severe thunderstorms to the Desert Southwest and tornado activity to Oklahoma, while a dust storm in its wake caused visibility to plummet Saturday in parts of Texas.

Now the main low-pressure area is transferring its energy offshore into a coastal system, also known as a nor'easter, that will intensify and move northward up the coast.

Winter storm warnings for heavy snow were in effect from the mountains of North Carolina through southwestern Connecticut, while watches extend all the way to extreme-northern Maine.

Snow is likely to move into Philadelphia by 4 or 5 p.m. Sunday, with snowfall rates increasing rapidly as the offshore low-pressure system begins to strengthen. Moderate to heavy snow is likely there by late evening, with numerous computer models showing a band of extremely heavy snow forming between Philadelphia and Rhode Island from Sunday night into Monday.

There is a chance that milder marine air drawn inland by the counterclockwise winds around the low-pressure center may flip Philadelphia over to mixed precipitation, including sleet or freezing rain, during the wee hours of Monday morning, before precipitation changes to rain and temporarily ends. Snow will fall heaviest immediately before that changeover, with snowfall rates of at least an inch per hour.

In New York, snow will arrive by late evening Sunday, with heavy snow and possible thundersnow during the morning hours on Monday. Significant uncertainty exists regarding a potential changeover to a wintry mix or rain around noontime, however.

If that transition does occur, snowfall amounts would stack up to a bit more than a foot. But if the changeover doesn't occur, more significant accumulations would be likely in the Big Apple - perhaps on the order of 18 inches or even a few inches more.

The National Weather Service included New York in the level 5 out of 5 "extreme impact" zone on their outlook map, citing the potential for "extreme disruptions to daily life."

Most of the surrounding Tri-State area can anticipate "major impacts" from the storm system, which looks to produce potentially blockbuster snowfall totals. In Boston, it's shaping up to be a more moderate event.

One particular high-resolution computer model, known as the North American Model (NAM), suggests that cold air would remain entrenched in New York for the duration of the event. While this simulation is a bit of an outlier, if that outcome were to come to fruition, this would wind up being a top-tier blockbuster storm for the city itself.

Such heavy snowfall could shut down city streets and cancel flights at the region's major airports.

"Despite the fact that we are forecasting up to 18 inches of snow, these numbers are conservative if you trust the NAM," wrote the National Weather Service in New York. "The NAM suggests that 2 ft is reasonable for this event where the heaviest band sets up and where it remains mostly snow."

If New York were to pick up 19.8 inches or more in 48 hours, it would qualify as one of the top 10 heaviest snowfalls on record in the city.

The top spot of 27.5 inches is held by the Jan. 22-23, 2016, storm.

Regardless, snowfall amounts will vary significantly depending on where the narrow corridors of heaviest snowfall, known as snow bands, set up and stall. In the heaviest bands, snowfall rates of one to two inches per hour or more are possible. Eastern Pennsylvania is also likely to see very heavy snowfall, including in such places as Harrisburg and Allentown, in the east and onto the western Philadelphia suburbs.

There's the possibility that places in northern New Jersey could see more than two feet of snow, as moisture from the Atlantic Ocean is drawn westward, directly into much colder air.

Snow will ease overnight Monday into Tuesday, but light snow is still likely most of Tuesday morning into the afternoon.

The jackpot of snow may occur in New York City proper, while also encompassing the southern Hudson Valley and most of central and eastern Pennsylvania.

In Hartford, Conn., the snow should stack up to six to 12 inches, with the potential for higher totals.

Farther northeast, in the Providence-to-Boston corridor, meteorologists were grappling with predicting the finicky rain-snow line. Snow will arrive there Monday midmorning, with the steadiest and heaviest precipitation occurring overnight before winding down midmorning Tuesday.

During the storm, the rain-snow line may wiggle back-and-forth near Interstate 95, with a touch of mixing possible in Boston and Providence, R.I., despite predominantly snow falling. For the South Shore, Upper Cape and Plymouth and Bristol counties in Massachusetts, a quick burst of snow will give way to mainly rain.

Storm totals of 10 to 14 inches are expected just west of Boston, with the biggest wild card being the amounts in Boston itself and along the immediate coastline.

The storm is even expected to spread snow into northern New England, benefiting ski areas in Vermont, New Hampshire and Maine.

Conditions will improve Wednesday before milder weather late in the week.

Published : February 01, 2021

By : The Washington Post · Matthew Cappucci, Andrew Freedman